Plasma generated in the air is conventionally used in various industrial fields, such as those dealing with welding of materials having a high melting point, surface cleaning in processes for semiconductor manufacture, improvement of the surface of metal materials and the like, and generation of fine particles. In addition, the application of plasma under ambient pressure has been expanding rapidly, so that plasma is now used in sterilizing processes for medical instruments.
As examples of a method for generating plasma under ambient pressure, generation of plasma using arc discharge and a method for heating a gas using microwaves can be cited.
Patent Document 1 discloses a method for generating plasma using arc discharge by applying a high-frequency voltage across electrodes, and in particular, discloses a method for using this plasma to form the tip of a syringe and at the same time carry out a sterilizing process.
Patent Document 1: Japanese Unexamined Patent Publication H6 (1994)-197930
The possibility of electrons and ions generated between electrodes colliding with the electrodes so that the electrodes become of a high temperature and wear out, as well as the possibility of part of the metal material forming the electrodes being released into the plasma so that an impurity gets mixed in with the plasma can be cited as being a problem with arc discharge.
Meanwhile, in methods for heating a gas for plasma supplied in a nonmetal pipe, for example a quartz pipe, using microwaves using a conductor placed around the nonmetal pipe, as shown in Patent Document 2, the microwaves applied to the conductor form an electrical field for excitation which penetrates the pipe, and thus, the gas is heated by the electrical field for excitation, so that it becomes of an ionized state, that is to say, so-called electroless discharge is possible, and the electrodes do not wear, and no impurity gets mixed in.
Patent Document 2: Japanese Unexamined Patent Publication 2004-172044
FIGS. 1(a) to 1(c) schematically show the plasma generator 100 in Patent Document 2. An antenna 105 for exciting a cavity and a loop antenna 106 for detecting an internal electromagnetic field are connected to a cavity 102 in coaxial form which surrounds a quartz pipe 101. An upper center conductor 103 which surrounds the quartz pipe 101 is placed in the top portion of the quartz pipe 101 and a lower center conductor 104 which surrounds the quartz pipe 101 in the same manner is placed in the bottom portion within the cavity 102 in a coaxial form.
FIGS. 1(b) and 1(c) are cross sectional diagrams showing the inside of the cavity 102 in coaxial form in FIG. 1(a), and the upper center conductor 103 is electrically connected to the inner surface of the cavity 102 in coaxial form at the upper end. In addition, an inner conductor 121 and an outer conductor 122 are engaged with a space in between in the lower center conductor 104, as shown in FIG. 1(b), and therefore, a choke structure is formed inside the lower center conductor 104, so that microwaves are prevented from being released to the outside. Furthermore, the lower end of the lower center conductor 104 is electrically connected on the inner surface of the cavity 102 in coaxial form.
Next, the operation of the plasma generator 100 is described. The height on the inside of the cavity 102 in coaxial form is set as a multiple of the half wavelength of the microwaves (in integers), and therefore, the microwaves inputted through the antenna 105 for exciting the cavity resonate inside the cavity 102 in coaxial form so as to form an electrical field for excitation 112 between the upper center conductor 103 and the lower center conductor 104, as shown in FIG. 1(b). The gas 110 which passes through the quartz pipe 101 is converted to plasma under the influence of this electrical field for excitation 112. The distribution of the electrical field is oscillation in TM mode.
When the gas inside the quartz pipe 101 is converted to plasma, the plasma functions in the same manner as a conductor, and thus, the orientation of the electrical field for excitation changes so that it lies in the direction from the inner wall of the cavity 102 in coaxial form to the plasma inside the quartz pipe 101, as shown by 113 in FIG. 1(c), so that the electrical field becomes of a coaxial mode (TEM mode), and subsequently, the electrical field for excitation 113 converts the gas within the quartz pipe 101 to plasma.
The change in the orientation of the electrical field for excitation before and after the ignition of the plasma changes the impedance inside the cavity 102 in coaxial form, so that the resonant frequency changes. In order to cope with this change in the frequency, the frequency of the microwaves is adjusted on the basis of the detection signal from the loop antenna 106 for detection of the internal electromagnetic field in Patent Document 2. In addition, it is suggested that a form which makes the change in the impedance be selected for the cavity.
In methods for creating a gap G between the two conductors placed around the quartz pipe at a distance from each other, as in Patent Document 2, however, the impedance inevitably changes before and after the ignition of the plasma, and thus, adjustment of the frequency of the applied microwaves becomes indispensable, as described above. Therefore, a mechanism for adjusting the frequency is required, making the entire apparatus complicated and raising the cost. In addition, in the case where a method for minimizing the change in the impedance by changing the form of the cavity 102 is adopted (here, Patent Document 2 does not disclose any concrete configuration), the form of the cavity 102 is limited, and it becomes difficult to flexibly cope with various changes, for example when a number of quartz pipes are provided.